The present invention relates to a method for realizing an apparatus for detecting decrease in an inner pressure of a tire with excellent accuracy at a low cost.
JP-A-2005-186739, JP-A-2003-146037 and JP-A-2003-94920 disclose a method of detecting decrease in an inner pressure of a tire from a vehicle speed calculated from GPS information, a distance, and a tire rotational number.
However, in order to detect decrease in an inner pressure of a tire from the vehicle speed calculated from GPS information and the tire rotational speed, it is necessary to detect change of the tire rotational speed only by decreasing the inner pressure. Namely, it is necessary to exclude the change of the rotational speed caused by a running state of a vehicle.
JP-A-2003-94920 discloses a method for detecting decrease in an air pressure of a tire by comparing a running track of a vehicle calculated from information of tire rotation with a running track of a vehicle obtained from the vehicle positioning information using such as GPS, but it does not indicate that the change of the rotational speed caused by the running state of a vehicle is excluded.
JP-A-2005-186739 and JP-A-2003-146037 specify the running state from the comparison of rotation states of the mutual tires, but when decrease in an inner pressure of a tire occurs, the relation of the mutual rotational speeds of four wheels already loses a balance, and an accurate running state cannot be specified.
Further, a method of detecting decrease in an inner pressure of a tire by calculating the tire dynamic load radii from the movement distance calculated from GPS information and the tire rotational numbers is known, but since the exclusion of the change of rotational numbers caused by the running state of a vehicle is inadequate, accuracy is inadequate.